The present invention relates generally to apparatus for intermittently advancing a web of flexible film, and for treating the web in a treating station, the treating station including a welding bar and sealing pad for forming seals in the web structure, along with a cutting blade for severing the web where desired. The drive means is arranged to impart repeating cycles of intermittent motion and dwell to the web, with the drive means being operated in synchronism with the welding or sealing apparatus. Means are provided to place the web under tension during the cutting operation, along with means to eliminate tension prior to and during the sealing operation to permit relaxation of the film prior to formation of the seal. Also, the draw rolls which are utilized in the web advancing mechanism are arranged to move through an initial short duration reversal prior to each web advance in order to break away any bond which may occur between the thermoplastic web and the sealing pad.
The apparatus of the present invention is particularly adapted for the preparation of bottom-seal bags which are formed from tubular webs of thermoplastic material, with the completed bag structure having a thermally welded bottom seal, along with an open top. The apparatus of the present invention controls the motion of the web through the working station, and controls this motion in such a way so as to relax the film prior to sealing and to break away any bond which may form between the web and the sealing pad.
In the past, various techniques have been proposed for bottom seal bag-making machines, these machines utilizing, for the most part, means for advancing the webs, along with welding or sealing means and cutting blades. In order to perform welding or sealing operations along with cutting operations, various precautions had to be taken in order to properly prepare the web for the specific operations to be employed. In order to form a uniform straight cut in the web, it is necessary that the web be under tension, and that the tension be maintained during the cutting operation. On the other hand, the sealing operation requires the web to be in a slack condition, preferably relaxed, since the welding or sealing operation softens the body of the thermoplastic material sufficiently to permit rupture to occur if any tension is applied during sealing. Furthermore, in order to preserve the condition of the seal subsequent to its formation, it is essential that means be provided to cool the sealed area prior to the time that any attempts are made to move the web. Unfortunately, when the sealed area was permitted to remain in contact with the sealing pad during cooling, the seal or sealed zone would occasionally form a bond with the surface of the sealing pad, and unless precautions are taken to release this film bond prior to movement of the web, jamming could occur.
The present invention obviates these problem areas by maintaining the web under tension prior to and during the cutting operation, permitting the web to relax prior to sealing and maintaining the web in a slack condition for the sealing operation, the sealing operation being performed subsequent to the cutting and relaxation operation. Thereafter, the seal is permitted to cool and positive web motion occurs in order to release the web from the surface of the sealing pad.